The Öresund (the Sound), which is a part of the Danish straits, is
linking the marine North Sea and the brackish Baltic Sea. It is a transition
zone where ecosystems are subjected to large gradients in terms of salinity,
temperature, carbonate chemistry, and dissolved oxygen concentration. In
addition to the highly variable environmental conditions, the area is
responding to anthropogenic disturbances in, e.g., nutrient loading,
temperature, and pH. We have reconstructed environmental changes in the
Öresund during the last ca. 200 years, and especially dissolved oxygen
concentration, salinity, organic matter content, and pollution levels, using
benthic foraminifera and sediment geochemistry. Five zones with
characteristic foraminiferal assemblages were identified, each reflecting
the environmental conditions for the respective period. The largest changes
occurred around 1950, when the foraminiferal assemblage shifted from a low
diversity fauna dominated by the species Stainforthia fusiformis to higher diversity and
abundance and dominance of the Elphidium species. Concurrently, the grain-size
distribution shifted from clayey to sandier sediment. To explore the
causes of the environmental changes, we used time series of reconstructed
wind conditions coupled with large-scale climate variations as recorded by
the North Atlantic Oscillation (NAO) index as well as the ECOSMO II model
of currents in the Öresund area. The results indicate increased changes
in the water circulation towards stronger currents in the area after the
1950s. The foraminiferal fauna responded quickly (<10 years) to
the environmental changes. Notably, when the wind conditions, and thereby
the current system, returned in the 1980s to the previous pattern, the
foraminiferal assemblage did not rebound. Instead, the foraminiferal faunas
displayed a new equilibrium state.

The Öresund (the Sound) is one part of the Danish straits between Sweden
and Denmark. Together with the Great and Little belts, they link the
open-ocean waters of the North Sea and the brackish waters of the Baltic
Sea. The confluence of the water masses creates a north–south gradient as
well as a strong vertical stratification of the water in terms of salinity,
carbonate chemistry, and dissolved oxygen concentration ([O2])
(Leppäranta and Myrberg, 2009). The depth of the halocline
mainly depends of the outflows from the Baltic Sea; a strong thermocline
develops during spring and summer, which further strengthens the vertical
stratification. Thus, the ecosystems in the Öresund are exposed – and
adapted – to a unique transitional environment. The region is also
characterized by intense human activities, with 4 million people living in
the vicinity of the Öresund and 85 million people living in the
catchment area of the Baltic Sea (HELCOM, 2009). Discharge from agriculture,
industry, and urban areas on both the Swedish and Danish sides of the
strait and the considerable impact of marine traffic – the strait is one
of the busiest waterways in the world – generate pollution and
eutrophication of the water (HELCOM, 2009; ICES 2010). Since
the 1980s, the implementation of efficient wastewater treatment and
measures in agriculture contributed to markedly reducing the amount of
nutrients coming from river run-off
(Nausch
et al., 1999; Carstensen et al., 2006; Rydberg et al., 2006). However, these
efforts in decreasing nutrient loads have not resulted in improved water
quality, due to the long timescales of biogeochemical cycles in reaching
equilibrium in the Baltic Sea region (Gustafsson et
al., 2012). The Öresund, like most of the Baltic Sea, is still assessed
to be eutrophic, and hypoxic events are frequent
(Rosenberg
et al., 1996; Conley et al., 2007, 2011; HELCOM, 2009; Wesslander et al., 2016).
Moreover, increasing temperatures and declining pH, linked to global climate
change and ocean acidification, have been reported for surface and bottom
waters in the area (Andersson et al., 2008;
Göransson, 2017). As a result, ecosystems in the Öresund are
currently under the combined impact of natural and anthropogenic stressors
(Henriksson, 1969; Göransson et al., 2002; HELCOM,
2009; ICES, 2010). The multiple
stressors currently affecting the environment make this region particularly
interesting to study and also highlight the need to obtain records of
decadal and centennial environmental changes. As noted above, both recent
human-induced impacts and climate variability have been substantial in the
region. Therefore the question arises whether these factors have affected
the benthic environment. Furthermore, sediment records of past environmental
changes can provide a crucial context for ongoing and future predicted changes
in the Öresund and Baltic Sea regions.

We used the marine sediment record and its contents of foraminifera as well
as sediment geochemistry to obtain records of decadal environmental changes.
Benthic foraminifera are widely used for environmental reconstructions,
based on their rapid response to environmental changes, broad distribution,
high densities, and often well-preserved tests (shells) in the sediment
(e.g., Sen Gupta, 1999b; Murray, 2006). For
instance, distributions of benthic foraminifera have been used for historical
environmental reconstructions of fjords on decadal to centennial timescales
on the Swedish western coast
(Nordberg
et al., 2000; Filipsson and Nordberg, 2004a, b; Polovodova Asteman and
Nordberg, 2013; Polovodova Asteman et al., 2015) and in the Kattegat
(Seidenkrantz, 1993; Christiansen et al., 1996).
In the Öresund, living foraminiferal assemblages have been
studied (Hansen, 1965;
Charrieau et al., 2018), but to the best
of our knowledge, no studies of past foraminiferal assemblages have been
performed. The objective of this study was to reconstruct the environmental
conditions of benthic systems during the last 2 centuries in the
Öresund by using foraminiferal fauna analysis in combination with
sediment geochemistry and grain size. Furthermore, we analyzed long time
series of wind conditions in the area to evaluate the coupling between local
changes in ecosystem variables and variations in atmospheric and subsequent
hydrographic conditions, and a possible link with large-scale variations
expressed through the North Atlantic Oscillation (NAO) index. Finally, we
compared our data with the ECOSMO II model
(Daewel and
Schrum, 2013, 2017) of currents and water circulation changes in the
Öresund area during the period 1948–2013.

The Öresund is a 118 km long narrow strait (Fig. 1). The water depth
in the northern part is on average 24 m, but it reaches 53 m south of the
island of Ven. The Öresund is an important link between the North Sea,
Skagerrak, Kattegat, and the Baltic Sea (Fig. 1), and up to 30 % of the
water exchange in the region goes through the Öresund
(Sayin and Krauß, 1996;
Leppäranta and Myrberg, 2009). The remaining part goes through the Great
and Little belts. The width of the Öresund varies between 4 and 28 km,
and the water has overall high current velocities, up to 1.5 m s−1 at
the upper water layer in the northern part (Nielsen,
2001). The fully marine Skagerrak consists of water masses from the North
Sea and the North Atlantic and in general a thin surface layer with water
originating from the Baltic Sea and rivers draining into the sea; the water
circulation forms a cyclonic gyre
(cf. Erbs-Hansen et al., 2012). Parts
of the Skagerrak waters reach the Kattegat and the Baltic Sea, where they
are successively diluted with the large amounts of freshwater (around 15 000 m3 s−1, Bergström and Carlsson, 1994) draining into
the Baltic Sea from numerous large rivers. The low-saline Baltic Sea surface
water is transported by the Baltic Current, which is typically confined
along the Swedish western coast in the Kattegat but which may cover a larger surface
area towards the west, depending on wind direction. The Baltic Current later
joins the Norwegian Coastal Current in the Skagerrak (Fig. 1). The large
freshwater input and the subsequent large salinity difference between the
Kattegat and Baltic Sea result in a two-layer structure in the Öresund
(Fig. 2) (She et al., 2007;
Leppäranta and Myrberg, 2009). The water stratification is influenced by
the surface water from the Arkona Basin (salinity 7.5–8.5), the surface water
from the Kattegat upper layer (salinity 18–26), and the lower layer of the
Kattegat (salinity 32–34).

Salinity, temperature, pH, [O2], and nutrient content, here represented
by dissolved inorganic nitrogen concentration [DIN] (nitrate + nitrite + ammonium), in the surface and bottom waters of the Öresund vary
seasonally (Fig. 3, Supplement Fig. S1). At the surface and bottom water,
salinity ranges between ∼8 and ∼18 and between
∼29 and ∼34, respectively, and it is more
stable between April and July, when the stratification is strongest
(Fig. 3). Temperature ranges between ∼1∘C in
February and ∼19∘C in July in the surface water,
while in the bottom water, the lowest temperature is found in March–April
with ∼5∘C, and the highest temperature in
October–November with ∼13∘C. The pH varies
between ∼8.1 and ∼8.6 in the surface water,
and between ∼7.8 and ∼8.6 in the bottom water,
without a clear seasonal pattern (Fig. 3). [O2] in the bottom water
reaches ∼7 mL L−1 in January, and it is typically below
2 mL L−1 in October, approaching hypoxic values. In the surface water,
[DIN] can reach ∼7µmol L−1 in January, and it is
∼0µmol L−1 between April and August (Fig. 3).

Figure 3Seasonal variability of salinity, temperature, pH, and dissolved
inorganic nitrogen (DIN) concentration at the surface water (light grey),
and seasonal variability of salinity, temperature, pH, and dissolved oxygen
concentration at the bottom water (40–50 m) (dark grey) of the Öresund.
The data were measured between 1965 and 2016 by the SMHI (Swedish
Meteorological and Hydrological Institute) at station W LANDSKRONA. The
number of measurements is indicated for each month.

3.1 Sampling

A suite of sediment cores, as well as water samples from the water column,
were collected in November 2013 during a cruise with R/V Skagerak. Here we present
the data from two sediment cores sampled at Öresund station DV-1
(55∘55.59′ N, 12∘42.66′ E) (Fig. 1), north of the
island of Ven. The water depth was 45 m, and CTD (conductivity, temperature,
depth) casts were taken to measure salinity, temperature, and [O2] in
the water column. Water samples were collected at 10, 15, 20, 30, and 43 m
from the Niskin bottles for carbonate chemistry analyses. The CTD and
carbonate chemistry data are presented in
Charrieau et al. (2018). In general, it
is challenging to obtain sediment cores in the Öresund, due to the high
current velocities up to 1.5 m s−1 (Nielsen,
2001), human-induced disturbances, and limited areas of recent sediment
deposition (Lumborg, 2005), but our site north of
Ven represents an accumulation area. The cores (9 cm inner diameter) were
collected using a GEMAX twin-barrel corer. The corer allowed sampling of 30
and 36 cm long sediment cores (referred to in this study as cores DV1-G and
DV1-I, respectively), which were sliced into 1 cm sections. The
samples from the DV1-G core were analyzed for carbon and nitrogen content and
grain size distribution, and were dated using Gamma spectroscopy. The samples
from the DV1-I core were analyzed with respect to foraminiferal fauna and
carbon and nitrogen content. The distinct carbon content profiles, measured
on both cores, were used to correlate the 210Pb dated DV1-G core with the
DV1-I core used for foraminiferal analyses.

3.2 Chronology

The age–depth model was established using 210Pb and 137Cs
techniques on samples from the DV1-G core. The samples were measured with an
ORTEC HPGe (High-Purity Germanium) Gamma Detector at the Department of
Geology at Lund University, Sweden. Corrections for self-absorption were
made for 210Pb following Cutshall et al. (1983).
The instruments were calibrated against in-house standards and the maximum
error was 0.5 years in the measurements. Excess (unsupported) 210Pb was
measured down to 23 cm and the age model was calculated based on the
Constant Rate of 210Pb Supply (CRS) model (Appleby,
2001).

3.3 Foraminifera analyses

Approximately 10 g of freeze-dried sediment per sample was wet-sieved
thought a 63 µm mesh screen and dried on filter paper at room
temperature. Subsequently, the samples were dry-sieved through 100 and
500 µm mesh screens and separated into the fractions 100–500 µm and > 500 µm. The foraminifera from every second
centimeter of the core – plus from additional centimeters around key zones –
were picked and sorted under a Nikon microscope (22 samples in total). A
minimum of 300 specimens per sample were picked and identified, as
recommended by Patterson and Fishbein (1989). If necessary the
samples were split with an Otto splitter (Otto, 1933). For
taxonomy at the genus level, we mainly followed Loeblich and
Tappan (1964) with some updates from the more recent literature, e.g.,
Tappan and Loeblich (1988). For taxonomy at the species
level, we mainly used Feyling-Hanssen (1964),
Feyling-Hanssen et al. (1971), and Murray and
Alve (2011). For original descriptions of the species, see Ellis
and Messina (1942 and supplements up to 2013).

Recently, the eastern Pacific morphospecies Nonionella stella has been presented as an
invasive species in the Skagerrak–Kattegat region
(Asteman and Schönfeld, 2016). However, a
comparison of N. stella DNA sequences from the Santa Barbara Basin (USA)
(Bernhard et al., 1997) with the Swedish western coast
specimens demonstrates that they represent two closely related species but
are not conspecific (Deldicq et al., 2019). Therefore, we have referred
to the species found here as Nonionella sp. T1,
following Deldicq
et al. (2019). The species Verneuilina media (here referred to as the genus Eggerelloides), which has often
been reported in previous studies from the Skagerrak–Kattegat area (e.g.,
Conradsen et al., 1994), was morphologically close to
Eggerelloides scabrus in the present material, and these two species have been grouped as E. medius/scabrus. The
taxon Elphidium excavatum forma clavata (cf. Feyling-Hanssen, 1972)
was referred to as Elphidium clavatum
following Darling
et al. (2016). Elphidium clavatum and Elphidium selseyense (Heron-Allen and Earland, 1911) were morphologically
difficult to separate in this region, as transitional forms occur. The
dominant species was E. clavatum, but we acknowledge that a few individuals of E. selseyense could
have been included in the counts. The taxon Ammonia beccarii was referred to as Ammonia batava, following
recent molecular work done on the taxon Ammonia in the Kattegat region
(Groeneveld
et al., 2018; Bird et al., 2019).

Foraminiferal density was calculated and normalized to the number of
specimens per 50 cm3. Data of densities for the first 2 cm
of the core are from Charrieau et al. (2018). Some specimens displayed decalcified tests; however, the inner
organic linings were preserved. These inner organic linings were reported
separately and not included in the total foraminiferal counts. Benthic
foraminiferal accumulation rates were calculated as follows:

BFAR(number of specimenscm-2yr-1)=BF×SAR,

where BF is the number of benthic foraminifera per cm3 and SAR is the
sediment accumulation rate (cm yr−1). Foraminiferal species that
accounted for > 5 % of the total fauna in at least one of the
samples were considered major species, and their density was used in
statistical analysis. The Shannon index was calculated to describe the
foraminiferal diversity. To determine foraminiferal zones, stratigraphically
constrained cluster analysis was performed, using the size-independent
Morisita index to account for the large differences in the densities
between samples (e.g., Krebs, 1998). A dendrogram was then
constructed based on arithmetic averages with the UPGMA method (unweighted
pair group method with arithmetic mean). Correspondence analysis was also
performed to determine significant foraminiferal species in each zone.
Statistical analyses were performed using the PAST software
(Hammer et al., 2001).

3.4 Organic matter analyses

Total organic carbon (TOC) and total nitrogen (TN) contents were measured for
both DV1-G and DV1-I. Approximately 8 mg of freeze-dried sediment was
homogenized for each centimeter and placed in silver capsules. Removal of
inorganic carbon was carried out by the in situ acidification (2M HCl) method
based on
Brodie
et al. (2011). TOC and TN contents were analyzed on a Costech ECS 4010
Elemental Analyzer at the Department of Geology, Lund University. The
instrument was calibrated against in-house standards. The analytical
precisions showed a reproducibility of 0.2 % and 0.03 % for TOC and TN
contents, respectively. The molar C∕N ratio was calculated.

3.5 Grain-size analyses

Grain-size analyses were performed on core DV1-G using 3.5 to 5 g of
freeze-dried sediment for each centimeter. Organic matter was removed by
adding 15 mL of 30 % H2O2 and heating during 3 to 4 min
until the reaction ceased. After the samples had cooled down, 10 mL of 10 % HCl was added to remove carbonates; thereafter the sediment was washed
with milli-Q until its pH was neutral. In the last step, biogenic silica was
removed by boiling the sediment in 100 mL 8 % NaOH and then washed until
neutral pH was reached. The sand fraction (> 63 µm) was
separated by sieving and the mass fraction of sand of each sample was
calculated. Grain sizes < 63 µm were analyzed by laser
diffraction using a Sedigraph III Particle Size Analyzer at the Department
of Geology, Lund University. The data were categorized into three size
groups, < 4 µm (clay), 4–63 µm (silt), and 63–2000 µm (sand).

3.6 Climate data and numerical modeling

Data from the High Resolution Atmospheric Forcing Fields (HiResAFF) dataset
covering the time period 1850–2008 (Schenk and Zorita,
2012; Schenk, 2015) were used to study the variations of
near-surface (10 m) wind conditions during the winter half of the year
(October to March). The daily dataset can be downloaded from WDC Climate
(Schenk, 2017). Wind conditions over the
Öresund are represented by the closest grid point of HiResAFF at
55∘ N and 12.5∘ E. The North Atlantic Oscillation (NAO)
index as defined by Jones et al. (1997)
for boreal winter (December to March) was used, with updates taken from the
Climate Research Unit (CRU, https://crudata.uea.ac.uk/cru/data/nao/, last access: January 2019). To allow comparison, the NAO and
wind data were normalized relative to the period 1850–2008. Changes in the
currents through the Öresund and the Kattegat were taken from the ECOSMO II fully
coupled physical biogeochemical model
(Daewel and
Schrum, 2013, 2017), which was forced by NCEP/NCAR reanalysis data and covers
the period 1950–2013. In model ECOSMO II, the simulated south–north
currents are represented as the VAV (vertically averaged V component) and the
simulated west–east currents as the UAV (vertically averaged U component).

4.1 Age model

The unsupported 210Pb showed a decreasing trend with depth in the DV1-G
core (Fig. 4a, b). The peak observed in the 137Cs around 9 cm
corresponds to the Chernobyl accident in 1986 (Fig. 4c). The
unsupported 210Pb allowed direct dating of the core between 2013 and
1913. The sedimentation rate ranged between 1 and 5.6 mm yr−1, with an
average of 2.2 mm yr−1, and decreased with depth. The ages of the
lower part of the sediment record were deduced by linear extrapolation based
on a sedimentation rate of 1.4 mm yr−1, corresponding to the linear mean
sedimentation rate between the years 1913 and 1946 (Fig. 4d).

Figure 4Age–depth calibration for the sediment sequence from the
Öresund (DV-1). (a) Total and supported 210Pb activity. (b)
Unsupported 210Pb activity and the associated age model. (c)137Cs
activity. The peak corresponds to the Chernobyl reactor accident in 1986. (d)
Age–depth model for the whole sediment sequence based on 210Pb dates
and calculated sediment accumulation rates (SARs).

4.2 Foraminiferal assemblages and sediment features

The foraminiferal assemblages were composed of 76 species from the
porcelaneous, hyalines, and agglutinated forms (0.3 %, 54.5 %, and 45.2 %,
respectively) (Supplement Table S1). Eleven foraminiferal species had relative
abundance higher than 5 % in at least one sample and were considered
major species (Plate 1, Fig. 5).

The cluster analysis revealed three main foraminiferal zones (FOR-A, FOR-B,
and FOR-C) (Figs. 5, 6). The correspondence analysis resulted in three
factors explaining 92 % of the variance, and in assemblages consisting of
seven significant species, presented in order of contribution: Nonionella sp. T1,
Nonionoides turgida, Ammonia batava, Stainforthia fusiformis, Elphidium albiumbilicatum, E. clavatum, and Elphidium magellanicum (Table 1). Based on both the cluster and the correspondence
analyses, five subzones could be separated to which we assigned dates
according to the age model: FOR-A1 (1807–1870), FOR-A2 (1870–1953), FOR-B1
(1953–1998), FOR-B2 (1998–2009), and FOR-C (2009–2013) (Figs. 5, 6).

Table 1Significant foraminiferal species and scores according to the
correspondence analysis.

4.2.1 Zone FOR-A1 (1807–1870)

The foraminiferal accumulation rate (BFAR) was on average 5±3
specimens cm−2 yr−1 in zone FOR-A1 (Fig. 5). The Shannon index
was stable and low, around 1.77±0.1 (Fig. 5). The agglutinated
species Eggerelloides medius/scabrus and the hyaline species Stainforthia fusiformis made major contributions to the
assemblages (relative abundances up to 53 % and 34 %, respectively;
Fig. 5a). Ammonia batava, the three Elphidium species (E. albiumbilicatum, E. clavatum, and E. magellanicum), Nonionellina labradorica, and the agglutinated species
Reophax subfusiformis were also major species, with abundances up to 7 %. The TOC andC∕N
values in this period were stable and were on average 3.36 % and 8.8 %, respectively (Fig. 7). The clay size fraction dominated the sediment
at the end of this period with a mean value of 63 %, and the sand content
was around 7 % (Fig. 7).

4.2.2 Zone FOR-A2 (1870–1953)

The BFAR was on average 9±5 specimens cm−2 yr−1 in zone
FOR-A2 (Fig. 5). The Shannon index was stable and low, around 1.94±0.15 (Fig. 5). Stainforthia fusiformis dominated the assemblage with relative abundances up to 56 % and a BFAR up to 608 specimens cm−2 yr−1 (Fig. 5a, b), which
is the highest BFAR observed for this species along the core. Egerelloides medius/scabrus was still very
abundant, up to 48 % (Fig. 5a). Ammonia batava, the three Elphidium species, and N. labradorica were present,
but with lower abundances than in zone FOR-A1 (maximum 5 %).
Bulimina marginata started to be more abundant, with an average relative abundance of 2 % in
the zone. Reophax subfusiformis was still a part of the assemblage and ranged between 1 % and 8 %. The TOC and C∕N values were stable and were on average 3.5 % and
8.74 %, respectively (Fig. 7). The clay size fraction dominated the
sediment during this period with a mean value of 63 %, and the sand
content was around 6 % (Fig. 7).

4.2.3 Zone FOR-B1 (1953–1998)

The BFAR increased massively during zone FOR-B1, with on average 54±31 specimens cm−2 yr−1 and with a peak at 93
specimens cm−2 yr−1 around 1965 (Fig. 5). It is lower during the
second part of the zone. The Shannon index was higher than in previous zones,
and it progressively increased towards the top of the zone (Shannon index
average 2.34±0.3) (Fig. 5). The highest BFARs along the core were
observed for all the dominant species of the previous zone, FOR-A2, except
for S. fusiformis (Fig. 5b). The zone was then also characterized by a drastic drop in
the relative abundance of S. fusiformis from 31 % to 2 % (Fig. 5a). Eggerrelloides medius/scabrus gradually
decreased in the zone, with relative abundances from 49 % to 24 %. The
highest relative abundance of A. batava for the entire record was in this zone, but it
was slowly decreasing as well, from 10 % to 3 %. The Elphidium species were more
abundant than in the FOR-A zones and their relative abundance was
increasing, especially for E. clavatum (increasing up to 23 %). Bulimina marginata, N. labradorica, and R. subfusiformis had a
relative abundance between 2 % and 6 %. A period of lower TOC values was
observed during zone FOR-B1 between 1953 and 1981, with an average of 2.38 % (Fig. 7). In the same period, the sand content showed a pronounced
increase, with an average of 24 % (Fig. 7).

4.2.4 Zone FOR-B2 (1998–2009)

In zone FOR-B2 the BFAR was still high, on average 55±6
specimens cm−2 yr−1 (Fig. 5). The Shannon index was high, with an
average of 2.8±0.2 (Fig. 5). The dominant species in the zone were
E. clavatum (up to 25 %) and Eggerelloides medius/scabrus (up to 15 %; Fig. 5a). The other two Elphidium species
reached their highest relative abundances over the core (up to 6 %).
Nonionella sp. T1, which had not occurred in the record until now, appeared in this
zone with a relative abundance of 1 %. Nonionoides turgida, which was present in very low
abundances along the core, had a mean abundance of 1 % in the zone
(Fig. 6a). Stainforthia fusiformis was present with up to 9 % in relative abundance and a BFAR
higher than in zone FOR-B1 (up to 570 specimens cm−2 yr−1).
Ammonia batava, B. marginata, N. labradorica, and R. subfusiformis were present and ranged between 2 % and 8 %. The TOC values were
increasing, with on average 3.05 % (Fig. 7). The sediment was dominated
by the clay fraction that was increasing (mean value of 58 %), and the
sand content was around 17 % (Fig. 7).

4.2.5 Zone FOR-C (2009–2013)

The BFAR was lower than in the previous zones FOR-B1 and FOR-B2, with on average
21±5 specimens cm−2 yr−1 (Fig. 5). The Shannon index was
highest during FOR-C (Shannon index average 2.93±0.07) (Fig. 5). Nonionella sp. T1 was a dominant species in the zone, with a strong increase in
relative abundance (from 1 % to 14 %) and in BFAR (from 61 to 137
specimens cm−2 yr−1) (Fig. 5a, b). Elphidium clavatum and R. subfusiformis were also dominant
species, with abundances up to 13 %. Nonionoides turgida had its highest relative abundance
and BFAR over the core during the zone, with up to 9 % and 342
specimens cm−2 yr−1, respectively (Fig. 5a, b). Eggerelloides medius/scabrus had its lowest
relative abundance over the core (up to 9 %). Bulimina marginata, the other two Elphidium species,
N. labradorica and S. fusiformis, were still present (between 1 % and 6 %), while Ammonia batava was absent during the
zone. The TOC and C∕N values were on average 3.71 % and 8.17 %,
respectively (Fig. 7). The clay size fraction dominated the sediment with
a mean value of 66 %, and the sand fraction was 7 % (Fig. 7).

4.2.6 Inner organic linings

Decalcified specimens were few and ranged between 0 and 4
specimens cm−2 yr−1 with an average of 1
specimen cm−2 yr−1 (Fig. 5). They were observed throughout the
core and especially during zone FOR-B2, and the morphology of the remaining
inner organic linings allowed the identification of the taxon Ammonia (Plate 1).

4.3 Simulated data from the ECOSMO II model

The VAV (vertically averaged south–north current velocity) through the
Öresund from the ECOSMO II model showed a reversed pattern compared to
the UAV (vertically averaged west–east current velocity) through the
Kattegat (Fig. 8). Thus, higher VAV through the Öresund translates to
an increase in the east-to-west flow in the Kattegat (lower UAV), suggesting
a stronger outflow from the Baltic Sea. The VAV through the Öresund had
the lowest values around 1955 (Fig. 8), followed by a shift to very high
values, which dominated throughout 1960–1970. A comparable period with
increased outflow from the Baltic into the Kattegat re-occurred during the
period 1993–2000.

Our environmental interpretations of the foraminiferal assemblages were
based on the ecological characteristics of each major species (Table 2).
Based on our environmental reconstructions, we could infer environmental
changes regarding [O2], salinity, organic matter content, and pollution
levels. Furthermore, we linked local environmental changes to larger
atmospheric and hydrographic conditions.

5.1 1807–1870

All the major species found in this period are tolerant to low-oxygen
conditions, especially the two main species: S. fusiformis and E. medius/scabrus (Table 2). Stainforthia fusiformis is an
opportunistic species used to hypoxic and potentially anoxic conditions
(Alve, 1994), and E. medius/scabrus specimens have been found alive down to
10 cm in the sediment, where no oxygen was available
(Cesbron et al., 2016). Stainforthia fusiformis and N. labradorica are also able to denitrify
(Piña-Ochoa et al., 2010). The fact that
species tolerant to low-oxygen conditions dominated, and the presence of
species that have the capacity to denitrify, suggest that low-oxygen
conditions were prevailing during this period. Furthermore, S. fusiformis prefers organic-rich substrate and clayey sediment, which was measured in our core during
this time period (Fig. 7). The low species diversity, as indicated by the
low Shannon index in this section of the core, can sometimes be linked with
low salinity (Sen Gupta, 1999a). Most of the major
species found during this period, such as the Elphidium species R. subfusiformis and A. batava, tolerate lower
salinities and are typical of brackish environments (Table 2). The low
occurrence of B. marginata, a typical marine species, also suggests a salinity lower
than in the open ocean. However, the salinity was probably not below
∼30, which is the lower limit for N. labradorica and S. fusiformis, which were present
throughout the period (Fig. 5, Table 2). In summary, this period appears
to have been characterized by low [O2], high organic matter content,
and salinity around 30.

5.2 1870–1953

Stainforthia fusiformis largely dominated the assemblage during this period, which may suggest
even lower oxygen conditions than during the previous period. This would
also go along with the low species diversity, which is sometimes linked to
low salinity. In the Öresund, low salinity can be caused by less
influence of more saline marine waters from the Kattegat, and changes in the
water transport through the strait are a possible explanation for both lower
salinity and oxygen levels. However, the occurrence of the marine species
B. marginata suggests that the salinity was at least ∼30 (Table 2). Low
oxygen can also be associated with high organic matter contents, since
oxygen is consumed during remineralization of organic matter. However, the
TOC levels observed in our core in this zone were high, but not higher than
in the previous zone (Fig. 7). At the time of the industrial revolution,
the Öresund, like the Baltic Sea in general, was used as a sewage
recipient for a mixture of domestic and industrial wastes, industrial
cooling water, and drainage water
(Henriksson, 1968), and the amount of
marine traffic increased considerably during this time period. Across the
Baltic Sea, this notably caused increased deposition of heavy metals
(Borg and Jonsson, 1996). This diverse type of
pollution could have modified the water properties, for example regarding
the carbonate chemistry and pH. Indeed, this zone is characterized by the
presence of organic linings in the core (see also Sect. 5.6). Moreover,
heavy metals, fuel ash (black carbon), and pesticides have been demonstrated
to generally have a negative effect on foraminiferal abundance and diversity
(Yanko et al., 1999; Geslin
et al., 2002). Pollution and low oxygen concentration could explain the low
species BFAR and diversity as well as the dissolution of tests during this
period. Some species that were present, i.e., the agglutinated species E. medius/scabrus and
R. subfusiformis, are known to be tolerant to various kinds of pollution (Table 2).

5.3 1953–1998

The large increase in general BFAR from 1953 suggests either more favorable
growth conditions or significant deposition of transported specimens into
the area. The coarser grain size observed during this period indicates
possible changes in the current system, which could affect both growing
conditions and transport of specimens (Fig. 7). However, the dating of our
core showed continuous sediment accumulation without any interruption during
this period (Fig. 4). Moreover, all the new dominating species were
already present in the core, even if in lower relative abundances (Fig. 5a). This indicates that the BFAR increase is most likely not due to
specimen transport, but rather is a result of a change in substrate and
environmental conditions that became favorable for a different foraminiferal
assemblage. The higher foraminiferal diversity compared to previous periods
and the decrease in the relative abundance of S. fusiformis may indicate more oxic
conditions. Elphidium clavatum has been found in coarse sediment in the area
(Bergsten et al., 1996), and other species that tolerate
sandy environments and varying TOC dominated the assemblage, such as A. batava, the
other species in the Elphidium species, B. marginata, and E. medius/scabrus. Furthermore, anthropogenic activities
such as agricultural practices were intensified during this period until the
1980s, which resulted in increased nutrient loads and resulting
eutrophication (i.e., Rydberg et al., 2006). The increase in organic matter may have been beneficial for
foraminifera as a food source. Food webs and species interaction like intra-
and inter-competition might also have been modified, giving the advantage to
some species such as the Elphidium species to develop in these new environmental
conditions.

The temporal coincidence with the shifts seen in the sediment record and the
anomalous wind conditions suggest a notable change in the currents through
the Öresund (Figs. 8, 9). The simulated currents through the
Öresund confirm such an abrupt change characterized by a shift from very
limited outflow from the Baltic to the Kattegat before ∼1960
to more than a decade of high relative outflow (high VAV) from the
Öresund to the Kattegat and high current velocities (Fig. 8). While
the simulation only covers the period after 1950, the analysis of wind
conditions and the NAO index suggest that the anomalies in the current and
sediment pattern from ∼ mid 1950s might have been
unprecedented since at least the middle of the 19th century (Fig. 9).
The shift in local sediment properties and the shift to higher BFAR and
species diversity suggest a combination of anomalous currents during a
period of unusually negative NAO index and the abrupt first advection of
anthropogenic eutrophication from the Baltic Sea towards the Kattegat.
Consistent with our findings, long-term variations in large volume changes
in the Baltic Sea (LVC,
Lehmann
and Post, 2015; Lehmann et al., 2017), which are calculated from > 29 cm (∼100 km3) daily sea-level changes at
Landsort (58.74∘ N; 17.87∘ E) for 1887–2015, show an
unusual cluster of both more frequent and also larger LVCs during the
1970s to 1980s relative to the entire time period. Notably, this period
coincides with the most dramatic shift in foraminiferal BFAR and species
diversity as well as an increase in sand content. The period before the
“regime shift” of the 1950s to 1960s is dominated by very infrequent and
few large LVC events. After the shift, the 1990s show also very few or
partly no LVC events with generally record-low Major Baltic Inflow events.

Thus, during this period, the ecosystems were affected by both climatic
effects through sedimentation changes and human impact. At the end of the
period, after ∼1980, the general BFAR was lower during a
short time (Figs. 5, 9). This could be linked to the measures that were
taken in agriculture and water treatments in order to reduce the nutrient
discharge (Carstensen et al., 2006; Conley et al., 2007), which could have reduced the food input.
Interestingly, when the sedimentation pattern changes again and the sand
content decreases markedly (Fig. 7), the new species in the foraminiferal
fauna do not return to previous relative abundances as one could have
expected (Fig. 5a). This suggests that once the foraminiferal fauna was
established in the Öresund area after the ∼1953 shift, it
created a new state of equilibrium.

5.4 1998–2009

The foraminiferal assemblage in this zone was similar to the previous one,
with high BFAR, high diversity, and the Elphidium species as dominating species. This
period is, however, characterized by the appearance of two new major
species: N. turgida and Nonionella sp. T1. Nonionella sp. T1 is suggested to be an invasive species in the
region which arrived by ship ballast tanks around 1985 and rapidly expanded
to the Kattegat and Öresund (Asteman
and Schönfeld, 2016). According to our dated core, the species arrived in
the Öresund ∼2000 CE (Fig. 5). The species has also
been present on the southern coast of Norway since ∼2009
(Deldicq et al., 2019), but additional genetic analyses are necessary to have a better
overview of the species' origin and expansion. Nonionoides turgida is an opportunistic species
that prefers high levels of organic matter in the sediment, as observed in
our core during this period (Fig. 7). The increase in the S. fusiformis BFAR suggests
lower [O2] than in the previous zone, which was indeed a general trend
in the Danish waters during this time period
(Conley et al., 2007). The salinity was probably
marine during this period, as suggested by the high occurrence of the marine
species B. marginata (Fig. 5). This period was then characterized by low [O2],
high organic matter content, and open ocean salinity.

5.5 2009–2013

The ability of Nonionella sp. T1 to denitrify and its tolerance of varying environments
may explain its rapid increase during this period. The increase in N. turgida also
suggests higher levels of organic matter in the sediment. The dominance of
these two species and the lower BFAR compared to previous periods suggest
low oxygen levels. This period is thus characterized by low [O2], high
organic matter content, and open ocean salinity.

5.6 Dissolution

The inner organic linings of the taxon Ammonia were observed (in low numbers,
< 5 units) along the whole core, except in the top 2 cm
(Fig. 5). Inner organic linings of the taxa Ammonia and/or Elphidium were noticed in
previous studies among dead fauna in the region
(Jarke, 1961; Hermelin, 1987: Baltic Sea;
Christiansen
et al., 1996; Murray and Alve, 1999: Kattegat and Skagerrak; Filipsson and
Nordberg, 2004b: Koljö Fjord). Dissolution of calcareous foraminiferal
tests has been considered a taphonomic process, affecting the test of the
specimens after their death
(Martin,
1999; Berkeley et al., 2007). However, living decalcified foraminifera have
been observed in their natural environment in the southern Baltic Sea
(Charrieau et al., 2018) and
Arcachon Bay, France (Cesbron et al., 2016), proving
that test dissolution can also occur while the specimens live. In any case,
low pH and low calcium carbonate saturation are suggested as involved in the
observed dissolution (Jarke, 1961; Christiansen, et al., 1996; Murray and Alve,
1999; Cesbron et al., 2016; Charrieau et al., 2018). Test dissolution may
occur in all calcitic species, but only the organic linings of Ammonia were found
in our study, probably because these were more robust to physical stress
such as abrasion.

In this study, we described an environmental record from the Öresund,
based on benthic foraminifera and geochemical data, and we link the
results with reconstructed wind data, NAO index, and currents from a
hydrodynamic model. Five foraminiferal zones were differentiated and
associated with environmental changes in terms of salinity, [O2], and
organic matter content. The main event is a major shift in the foraminiferal
assemblage ∼1950, when the BFAR massively increased and S. fusiformis
stopped dominating the assemblage. This period also corresponds to an
increase in grain size, resulting in a higher sand content. The grain-size
distribution suggests changes in the current velocities which are confirmed
by simulated current velocity through the Öresund. Human activities
through increased eutrophication also influenced the foraminiferal fauna
changes during this period. Organic linings of Ammonia were observed throughout the
core, probably linked to low pH and calcium carbonate saturation, affecting
test preservation.

The long-term reconstruction of sediment and ecosystem parameters since
∼1807 suggests that the onset of increased anthropogenic
eutrophication of the eastern Kattegat started with an abrupt shift
∼1960 during a period of a strongly negative NAO index. With
unusually calm wind conditions during the winter half and increased easterly
winds, the conditions were ideal for larger Baltic outflow events, which is a
prerequisite for more frequent and stronger major Baltic inflow events
(Lehmann et al., 2017), as
calculated from LVC events during this period. Our high-resolution sediment
record points towards the importance of considering also large Baltic
outflow events for the Kattegat environment. Since the Baltic Sea is much
more eutrophic, less oxygenated, and less saline, large outflow events may
have a significant impact also on the Kattegat ecosystem. Periods with a
negative NAO or conditions with intense atmospheric blocking over
Scandinavia like in 2018 may also increase the influence of the Baltic Sea's
environmental problems on the Kattegat region.

The climate and numerical modeling data are accessible as described in the methods section. The foraminiferal data are available in Table S1. The hydrographic data used
in the projected are collected from SMHI's SHARK database. The SHARK data
collection is organized by the environmental monitoring program.

Figure S1, with time series of salinity, temperature, and dissolved oxygen
concentration at the bottom water of the Öresund, and Table S1, with
total foraminiferal faunas normalized to 50 cm3 along the DV core, are
available in the online version of the article. The supplement related to this article is available online at: https://doi.org/10.5194/bg-16-3835-2019-supplement.

HLF conceived and planned the work. HLF, LMC, and KL collected and analyzed the data. FS and UD carried out the simulations. LMC took the lead in writing the manuscript and creating the figures. All the authors provided critical feedback and helped shape the research, analysis, and manuscript.

We would like to thank the captain and the crew of R/V Skagerak. We acknowledge
Git Klintvik Ahlberg for the assistance in the laboratory, Yasmin Bokhari
Friberg and Åsa Wallin for the help with the grain-size analysis, and
Guillaume Fontorbe for help with the age model.

The SHARK data
collection is funded by the Swedish Environmental Protection Agency. This research has been supported by the Swedish Research Council, FORMAS (grant nos. 2012-2140 and 217-2010-126), the Royal Physiographic Society, and the Oscar and Lili
Lamm Foundation.

Patterson, R. T. and Fishbein, E.: Re-examination of the statistical
methods used to determine the number of point counts needed for
micropaleontological quantitative research, J. Paleontol., 63, 245–248, 1989.

Schenk, F.: The analog-method as statistical upscaling tool for
meteorological field reconstructions over Northern Europe since 1850,
Dissertation, University of Hamburg,
available at: http://ediss.sub.uni-hamburg.de/volltexte/2015/7156/, last access: 3 February 2015.

We reconstructed environmental changes in the Öresund during the last 200 years, using foraminifera (microfossils), sediment, and climate data. Five zones were identified, reflecting oxygen, salinity, food content, and pollution levels for each period. The largest changes occurred ~ 1950, towards stronger currents. The foraminifera responded quickly (

We reconstructed environmental changes in the Öresund during the last 200 years, using...